Alloy steel for internal combustion valves or valve elements



' Patented Oct. 24, 1939 UNITED STATES PATENT OFFICE Barry L. Frevertand Francis E. Foley, Philadelphia,'Pa., assignors to The MidvaleCompany,

Philadelphia,

Pa., a corporation of Delaware No Drawing. Application February 23,1938, Serial No. 191,966

8 Claims.

The problem of producing a steel suitable for use in the exhaust valvesof internal combustion engines is one which has long engaged the attention of metallurgists because of the constantly increasing severity ofthe" conditions of service. Ethyl lead in gasoline, higher compressionratios, increased temperatures, have each presented in turn a freshproblem for solution. Material satisfactory at one stage of developmenthas been made obsolete by the next advance in combustion engineering.

Many patents have been granted for compositions suitable for the valverequirements of their corresponding dates, but today automotiveengineers are demanding a new steel reasonable in cost, free fromobjectionable hot oxidation, free from pick-up, and strong enough atoperating temperatures to withstand the stresses of service, especiallythe tendency of valves to umbrella or flatten, with loss of sealingpower. A tensile strength of about 6000 pounds per square inch at 1600F. has been in the past considered quite satisfactory, and approximatelythis hot strength characterizes one of the valve steels in widest usetoday, but valve steels of such limitedhot strength, even thoughpossessing good resistance to scaling, do not satisfy even present dayrequirements and their abandonment as a valve steel in the near futureawaits only the development of a valve steel possessing a greatlysuperior hot strength without sacrifice of scaling resist ance.

Alloys of the essentially Cr-Mn type have heretofore been used almostexclusively for their physical properties and their resistance tostaining, at atmospheric temperatures; but their resistance to scalinghas been little investigated and their strength when exposed to hightemperatures not at all, so far as we are aware. It is in this fieldthat our invention lies.

An alloy steel of high, low and medium carbon.

content and containing chromium within a broad range of 10% to 45%,manganese within a broad range of 3% to 25%, with or without silicon upto 3%, with or without a small percentage of nickel, is disclosed in anexpired patent. Certain alloys containing constituents whose proportionswould respond to'the exceedingly broad ranges of the patent may be, asclaimed for them, re-

sistant to oxidation at high temperatures, or to stain, to be readilyforgeable and machinable and to have a high tensile strength relative tocast iron; but we are aware of no disclosure of any specificcompositions within such broad ranges that would have, in addition tothese qualities, the vital requirement of high strength at the elevatedoperating temperatures to which valves for internal combustion enginesare subjected. I

As an example of a chromium-manganese that 5 would respond to thesebroad ranges, there has been developed more recently an alloy steeladapted more particularly to the manufacture of articles made of sheetmetal and containing carbon less than 3%, chromium 16-22% and manganese6% to 14%, with or without nickel. This steel alloy is subjected to aheat treatment to produce a material having the great strength andtoughness essentialfor the deep drawing, cold,

of articles made of sheet steel. But it is wholly 15 deficient in thecombination of qualities required ,for valve steel.

While the resistance to scaling of alloys of the class above discussedis superior to that of less highly alloyed steel, they do not possesssufiicient strength for satisfactory continuous service as internalcombustion exhaust valves at the high temperatures of operation.Knowledge ,of such alloys does not teach how to secure a materialsuitable in both respects for use as internal combustion engine valves,and none of them discloses such proportioning of the alloyingingredients as would produce a steel having the characteristics nowdeemed necessary for such valves.

Annealing boxes have been made of a manganese-chromium-copper-aluminumalloy-a composition patented by us November 7, 1933, No.1,933,900-having sufiicient strength when hot, and sufiicient toughnesswhen cold, for the pur-- 86 pose intended, besides a high resistance tooxidation. For valves of internal combustion engines this composition isnot satisfactory, having too low hot strength combined with a tendencytowards red-shortness. This tendency towards 4o red-shortness can becorrected by substituting nickel for much or all of the copper, and wefind that a reduction in the amount of chromium present increases thehot strength, although at the loss of part of the scale resistanceunless means are taken to overcome it.

For various purposes, including valves and valve seats for internalcombustion engines, it is known to use a chrome-nickel ormanganesemolybdenum or tungsten alloy and a chrome- 60 nickel ormanganese-silicon-molybdenum or tungsten alloy, with proportions ofthese elements that shall produce a steel initially ferritic, as cast,rolled, forged or annealed. These alloys may be called high chromiumalloys, since they contemplate the use of a minimum of 18% chromium anda maximum as high as 35%. They are hardened by the simple operation ofheating without subsequent accelerated cooling. The proportions of theconstituent alloys have, however, been so adjusted as to respond tothese two conditions. The type of hardening to which the materialsrespond is that well-known as dispersion or precipitation hardening; Inthe case of steels, it is accompanied by a serious loss of toughnesswhich may limit or prevent uses for which the material would otherwisebe well suited.

There is also known a nickel-chromium-silicon steel for exhaust valves,with carbon .20-2.00, nickel over .65 and less than 4, chromium 10-25,and silicon from more than 1-6. This is also a ferritic or alpha steelwith a high critical temperature.

On the other hand, ferrous alloys wholly or predominantly austenitic bynature have critical temperatures below atmospheric and generally hardenbut little by simple heating; conversely, they tend to soften under theinfluence of heat much more slowly than do the usual ferritic alloys andthey maintain a very high degree of toughness. It is in this field ofalloys wholly or predominantly austenitic that our invention lies. Asimple way of differentiation between our alloys and the two types ofalloys last mentioned is that our alloys attract a magnet not at all orbut feebly, whereas the other alloys are all strongly magnetic.

We have found that sufficient hot strength can be obtained in anaustenitic steel containing medium chromium by the use of an appropriateamount of nickel or a less amount of nickel in connection with aconsiderably greater amount of manganese, nickel being a much moreeffective hardener than manganese. Both elements suffer in thisconnection from the same defectthat of causing a loss in scaleresistance of the alloy at high heat.

While it would seem that appropriate additions of silicon or chromium,or both, should correct this condition, our researches proved thateffective amounts of them, alone, seriously reduce the hot tensilestrength.

We have found that a small addition of molybdenum to a heat containingabout 15% chromium will maintain the cold tensile strength almostunimpaired, with but a slightly decreased resistance to scaling. If thechromium be of the order of 20%, the effect of molybdenum tends to bereversed and to reduce the hot strength.

Aluminum in suitable amount has been proved by us both to reduce theamount of scaling at heat and to maintain the hot strength, the lattereffect being unexpected.

We have developed different compositions containing chromium, manganese,silicon, and nickel, with or without addition of aluminum and/ormolybdenum, in proportions within comparatively limited ranges, each ofwhich produces an alloy steel which combines in high degree the majorrequirements hereinbefore specified. None ofthem, however, possesses allof these major requirements in the highest degree. The proportions ofthe several alloying constituents necessarily vary in accordance withthe qualities which,

in any particular case, it is sought to secure in' the highest degree,The present disclosure is therefore not intended to include all thecompositions which we have! developed. Theydo include, however, alloysteels which possess the quality of tensile strength-resistance to distortion-when exposed to high temperature, many times that characterizingcommonly used ferritic exhaust valve steels and a resistance to scalingwhich is in excess of that of valve steels which heretofore have beenaccepted as satisfactory; and they also include alloy steels whichpossess a maximum degree of resistance to scaling and a hot strengthwhich, although not of maximum degree, substantially exceeds commercialrequirements. All of them are readily machinable and hardenable.

The following is a preferred example of an alloy steel having maximumresistance to scaling with quite high hot strength:

(a) Carbon .48, chromium 14.5, manganese 3.5, nickel 2.2, silicon 2.6,aluminum 8, molybdenum .5.

If the proportions of chromium, manganese, silicon and nickel do notvary greatly from those above specified, either molybdenum or aluminumor both molybdenum and aluminum may be excluded from the composition andstill produce a valve steel quite equal to the other in scale resistantqualities and nearly its equal in hot strength. If only aluminum beexcluded, the hot strength will not bereduced at all andthe scaleresistantv qualities will not be reduced to a serious extent. I

If only molybdenum be excluded, scale resistance will not be reduced atall and while the hot strength will be somewhat decreased, it will stillremain quite high. If still greater hot strength be desired, it can besecured by increasing the proportion of molybdenum up to (say) 1.5,although in that case it. is advisable to retain the aluminum in aproportion of about one per cent.

' (d) Carbon .52, chromium 19.6, manganese 9.5,

silicon 3, nickel 2.2, molybdenum .5.

This composition gives a hot strength comparable to that characterizingthe preferred compositions, but with a reduced, yet still good,resistance to scaling.

For certain uses, and even for certain valve steels, it may be desirableto secure the highest possible hot strength when that quality, ratherthan the highest degree of scaling resistance, is the desideratum. Thefollowing are example compositions that fully meet this requirement:

(f) Carbon .43, chromium 10.44, manganese 9.3,

silicon 2.65, nickel 1.8, molybdenum 1.81.

(9) Carbon .5, chromium 18, manganese 10, silicon 3, nickel 3.

is even superior in tensile strength at high temperatures to composition(at) but with somewhat reduced scaling resistance. A reduction in thepercentage of nickel to about 2% and in the percentage of chromium toabout 15% gives a steel j equal to (g) in resistance to scaling andsomewhat superior to it in hot strength and which, being cheaper, ispreferable.

Generally speaking: If the chromium in the alloy be increased, the otherelements being held substantially constant, scaling resistance isimproved, but the hot strength falls. The hot strength, however, maystill be maintained high and quite sufficient where resistance toscaling is the prime requirement rather than strength to resist thedeformation occurring in valves of large diameter. The addition ofaluminum improves resistance to scaling without reducing the hotstrength. Molybdenum materially increases the hot strength. Silicon isnecessary in some substantial proportion if good scaling resistance isdesired, although less silicon is required if aluminum be added. Nickelcannot replace manganese, nor manganese nickel, but both are requiredand the manganese should be in higher proportion than the nickel. Insome compositions the proportion of nickel is permissibly very low. If arelatively high percentage of chromium be used,

it should be associated with a-higher percentage of manganese or nickel,but less silicon is permissible. If a relatively low percentage ofchromium be used, lower percentages of manganese and nickel suffice, buta higher percentage of silicon is required. With low chromium and highsilicon, the percentage of aluminum may be small and sometimes may beomitted. If it be desired to secure, not merely a very good, but anexceptionally high, resistance to heat, molybdenum cannot be omitted,but with increase in the percentage of chromium the percentage ofmolybdenum must usually be reduced.

It is to be understood from the foregoing that the compositions we havedeveloped range from those having great resistance to scaling andmoderate hot strength to those having great hot strength with a lesseneddegree of scale resistance. Within this range one skilled in the art canselect that combination which will best resist the conditions to be met.

A certain degree ofhardening, presumably by precipitation of carbides inthe austenite, can be obtained in those alloys essentially withoutaluminum, and with manganese in the lower range, by reheating to about1600 R; if the manganese be in the higher range, hardening takes placeat about 1400 F. with the production of a certain amount of magneticattraction.

With aluminum in appreciable quantities, say of the order of 1.5% ormore, there is a pronounced hardening at about 1000 F., presumably dueto the precipitation of such a compound as NiAl in a certain proportionof the grains rendered ferritic by local enrichment of aluminum. Suchhardening forms part of the subject-matter of Patent No. 1,943,595,applied for in 1931 by Foley, one of the present applicants, and grantedin 1934.

While in certain claims we have specified certain ranges of proportionsof the alloy constituents, it will be understood that we do not claimall compositions in which the proportions of the several constituentsrespond to the claim, but that such proportions must be so adjusted thatthe percentage of iron will be at least a'ndnot over 85%, which alsodefinitely excludes the use in any one composition of minimum or maximumproportions of all the other elements specified; and (which is ofprimary importance) the proportions must be so adjusted as to produce apredominantly austenitic steel. Merely varying the proportion of ironwill not sufllce. It is possible so tovary the proportions within theranges specified as to produce an initially predominantly ferritic steelor a steel which cannot be said to be predominantly ferritic oraustenitic; but such steels do not have the characteristic qualities ofour improved steel. With the aid of the examples given herein, and withthe knowledge of those skilled in the art, the production of apredominantly austenitic steel can be assured.

In our preferred composition, the proportion of chromium should notmaterially exceed 17% if the best combination of scaling resistance andhigh hot strength are sought. While the efiiciency of silicon to greatlypromote resistance to scaling is well known, its tendency to produce theferritic state is such that it should not be used in high proportion. Inour preferred composition the silicon should not exceed 3.5% and in somecompositions may be kept as low as 1.5% to 2.5% and yet sufficientlypromote resistance to scaling. The amount of silicon may be reduced asthat of aluminum is increased but it should not be below 1.25. Nickelincreases the hot strength, assists the manganese in making the steelaustenitic, improves the crystalline structure and tends to keep thescale adherent; but its use except in small proportion so decreasesresistance to oxidation that it should constitute only a minorproportion, preferably not exceeding 3%, and a much smaller proportionoften suflices. Manganese is quite essential to lower the criticaltemperature and render the steel austenitic. The proportion requiredvaries with the proportion of the other ingredients. and should rarelybe less than 3%. Molybdenum, which increases the hot strength, ispreferably not in greater proportion than about 2.5% and a smallerproportion, from .5% to 2% is usually adequate in those compositionswhich include it. Like that of silicon, the tendency of aluminum toproduce the ferritic state is undesirable, but its contribution tooxidation resistance issuch that its presence in small proportion, notover about 2%, preferably from .5% to l is desirable.

The percentage of carbon is not of substant al importance. It may varyfrom .10 to 1.25. The

illustrative compositions given herein and other austenitic steelsembodying our invention that we have tested indicate that the ranges ofthe other constituents must be as follows: Chromium 10 to 18%,preferably not over 17% if a very high hot strength is desired;manganese 3 to 11%, with manganese from 7 to 11% where maximum hotstrength is desired; nickel 1.5 to 4%; and silicon 1.25 to 4%.

It is necessary, in most compositions, to limit the proportion ofsilicon to within 4% and often to within 3% to insure the production ofa dominantly austenitic steel.

Molybdenum and aluminum, either or both, are permissive additions in anamount not exceeding 2.5% of either. The percentage of silicon should bebetween 2.5 and 4% if no aluminum be added and may be reduced to thespecified minimum of 1.25% only if aluminum be added in such proportionthat the two metals of the group are between 2.5 and 5%.

An important factor in the production of our improved alloy is that theproportion of chromium and nickel and (when used) of molybdenum andaluminum may be maintained so low that valves and valve seats forinternal combustion may beproduced at minimum expense and still meetpresent day exacting requirements.

Patent 182 1. An alloy steel having exceptional strength at hightemperature and resistance to scaling which contains carbon .10 to 1.0%,chromium over 10 and less than 20%, manganese and nickel 5 to 13%, themanganese being over 3% and less than 10.25% and the nickel being over1.75% and not over 3.5%, the percentage of manganese in any compositionsubstantially exceeding the percentage of nickel therein, and 2.5 to4.5% of metal of the group consisting of silicon and aluminum, thesilicon being over 1.25%; the balance of the composition beingsubstantially iron. v

.2. An alloy steel having exceptional strength at high temperature andresistance to scaling which contains carbon .10 to 1.0%, chromium overand less than 20%, manganese and nickel '5 to 13%, the manganese beingover 3% and less than 10.25% and the nickel being over 1.75% and notover 3.5%, the percentage of manganese in any composition substantiallyexceeding the percentage of nickel therein, and silicon 2.5 to 4%; thebalance of the composition being substantially iron.

3.'An alloy steel having high strength at high temperature and maximumresistance to scaling comprising carbon .10 to 1.0%, chronium notvarying more than 2% from 14.75%, nickel not varying over 1% from 2.5%,manganese varying less than 0.5% from 3.5%, the percentage of manganese'in any composition substantially exceeding the percentage of nickelthereimand 2.5 to 4% of metal of the, group consisting of silicon andaluminum, the silicon being over 1.25%; the balance of the compositionbeing substantially iron.

4. An alloy steel having high strength at high temperature and maximumresistance to scaling comprising carbon .10 to 1.0%, chromium notvarying more than 2% from 14.75%, nickel not varying over 1% from 2.75%,manganese varying less than 0.5% from 3.5%, the percentage of manganesein any composition substantially exceeding the percentage of nickeltherein, and silicon not varying more than 375% from 3.25%; the balanceof the composition being substantially iron.

5. A valve or valve element for internal combustion engines having thecomposition and characteristics set forth in claim 1.

6. A valve or valve element for internal combustion engines having thecomposition and characteristics set forth in claim 3.

7. The alloy steel defined in claim 1 in which the steel is dominatinglyaustenitic.

8. The alloy steel defined in claim 3, in which the steel isdominatingly austenitic.

HARRY L. FREVERT. FRANCIS B. FOLEY.

